What Is a CMOS Sensor?
A CMOS sensor is an image sensor used in digital cameras and other photographic equipment. The light received by the individual photodetectors is converted into an electric charge, which is then passed through an amplifier circuit composed of CMOS and extracted as a voltage or current according to the intensity of the light.
In the past, CCD sensors were used as the mainstream image sensor, featuring a structure in which the charge is transferred by the CCD and converted to a voltage via a floating diffusion amplifier (FDA).
CCD sensors have advantages over CMOS sensors in terms of sensitivity, signal-to-noise ratio, and low dark current, but they have disadvantages in terms of complex power supply configuration, unavoidable smear generation, and the fact that the manufacturing process is special and general CMOS LSI production equipment cannot be used. Recently, CMOS sensors have become the mainstay of image sensors due to advances in methods for reducing the effects of dark current and improving the signal-to-noise ratio in CMOS sensors.
Uses of CMOS Sensors
In the past, CMOS sensors were used in cameras mounted on smartphones and tablets because of their low cost of production. On the other hand, CCD sensors with low noise were mainly used in single-lens reflex cameras and video cameras, which require high image quality.
However, as noise reduction methods for CCD sensors evolved, the smear and blooming that had been a problem with CCD sensors did not occur, and the CCD sensor was gradually replaced by the CMOS sensors. CMOS sensors are now used as image sensors in all types of photographic equipment.
Principle of CMOS Sensors
The basic function of an image sensor is to store and transfer the electric charge generated by the light-receiving elements arranged in large numbers on its surface, convert it into a voltage or electric current, and output it. In this respect, CCD sensors and CMOS sensors share the same function.
The major difference between the two lies in the charge transfer mechanism: CCD sensors have a grid of photodiodes as light-receiving elements, and charge can be temporarily stored in the N-type region of these photodiodes.
A vertical CCD is placed adjacent to these photodiodes, and all the charges accumulated by each photodiode at a given time are simultaneously transferred to the vertical CCD. The charges are sequentially transferred and delivered to the horizontal CCD.
The horizontal CCD transfers the charge transferred from the vertical CCD to the FDA, which outputs a voltage corresponding to the amount of charge, thus providing a voltage output corresponding to the intensity of the light irradiating the photodiodes. As described above, in a CCD sensor, the amount of charge from all photodiodes is output sequentially.
On the other hand, CMOS sensors have a photodiode, an amplifier that amplifies the output of the photodiode, and a switch element that connects the amplifier output to the signal line, so that light reception, conversion, amplification, and output are performed for each photodiode.
From this configuration, CMOS sensors can specify individual photodiodes by combining horizontal and vertical scanning signals and can extract voltage or current according to the amount of charge. Thus, any photodiode can be selected and its signal be read out.
Due to these structural differences, CMOS sensors have the advantage of high-speed readout by limiting the signal to the necessary area and eliminating the transfer noise of CCDs. Furthermore, while CCD sensors inevitably suffer from smears caused by noise components flowing into the CCD, this is not the case with CMOS sensors.
CMOS Sensors Structure
CMOS sensors combine a photodiode, which is a light-receiving element, with an amplifier and switch elements, and integrate many of these elements. The photodiode manufacturing process is special and different from that of transistors, but the other components are identical to those of CMOS LSIs, so the use of CMOS manufacturing equipment is advantageous over CCDs.
New developments are also emerging about photodiode placement. In this structure, photodiodes are placed on the backside of the CCD, whereas circuits such as amplifiers and switch elements are formed on the front side. The photodiode is connected to the circuitry via internal wiring. Although the manufacturing process is more complex, the photodiodes can be placed with no gaps between them, which improves light collection efficiency.
The circuitry in CMOS sensors operates with a single power supply, so basically only a single power supply of about 3.3 V is required. CMOS sensors have an advantage in terms of power consumption.
Other Information on CMOS Sensors
1. CMOS Sensors Market share
Sony had a dominant market share when CCD sensors were at their peak, but now that CMOS sensors have become the mainstay and their primary application has shifted to smartphones, Sony’s market share is gradually declining. In 2021, Sony’s market share in terms of value will be 45%, Samsung’s 26%, and OmniVision’s 11%, according to the survey.
2. Size of CMOS Sensors
CMOS image sensors are available in a variety of sizes, from large to small. Taking Canon’s CMOS image sensors as an example, there are six different sizes of image sensors.
- 35mm full size (approx. 36mm x 24mm)
- APS-H size (approx. 29 mm x 19 mm)
- APS-C size (approx. 22 mm x 15 mm)
However, they are not sold to the general public and are limited to use for their cameras.
- 1 inch
- 2/3 inch
- 1/1.8 inch
Generally, for the same number of pixels, the larger the sensor size, the better the image quality. Also, the wider the aperture, the higher the sensitivity.